The present invention relates to distortion or error canceling amplifiers. More particularly, the invention relates to a distortion-canceling amplifier that has low spectral regrowth and enhanced power efficiency.
Modern digital communications systems provide high spectral efficiency, clarity, and fade resistance that is unmatched by older analog systems. Nevertheless, increasing consumer demand requires even better systems. To achieve further improvements in performance, communications systems such as CDMA (code division multiple access) and GSM (global system for mobile communication) will require amplifiers that provide relatively high-power signals with limited or minimal spectral regrowth. As is known, spectral regrowth refers to the amplification of signals outside a desired frequency range. Ideally, an amplifier would amplify signals without creating noise, particularly noise outside the frequency range of the input signal. In practice, this ideal has not yet been achieved, and spectral regrowth often causes interference between adjacent communication channels. Limiting or reducing spectral regrowth is an important factor to improving spectral efficiency. When spectral regrowth is low, interference is reduced. With reduced interference, channel separation may be narrowed and the number of channels in a given bandwidth may be increased.
Simple class A or class AB amplifiers have been used in communications applications. When using a class A or class AB amplifier, spectral regrowth can be controlled by operating a simple amplifier at 8 to 10 dB below compression (the point where the amplifier clips or saturates). However, simple class A and class AB amplifiers waste about 90% of the available output power.
Another type of amplifier, a feed-forward amplifier, can be operated at higher power levels with higher efficiency. Feed-forward amplifiers (xe2x80x9cFFAsxe2x80x9d) use two amplifiers: a main amplifier and a distortion-canceling amplifier. The main amplifier is operated at a relatively high power level and generates an amplified, but distorted or noisy signal. A feed-forward circuit or path is used to estimate the distortion generated by the main amplifier. The estimated distortion is inverted, amplified, and then summed with the output from the main amplifier to remove the distortion in the amplified signal. An exemplary feed-forward amplifier 10 is shown in FIG. 1.
The amplifier 10 receives an input signal 12. The input signal 12 is delivered to a coupler 14. The coupler 14 outputs a signal 16 to a gain and phase block 18. The output of the gain and phase block 18 is delivered to a main amplifier 20, which could be a class A or class AB amplifier. The main amplifier 20 amplifies the input signal 12 by a predetermined gain and outputs an amplified signal 21 to a coupler 22. The amplified signal 21 includes a main signal component (the amplified input signal 12) and a noise or error component. The gain and phase control block 18 and main amplifier 20 comprise a main signal path 24.
The coupler 14 outputs a second signal 26 that is delivered to a delay 28. The amount of time delay presented by the delay 28 is approximately equal to the time required for the signal 16 to propagate through the main signal path 24. The delayed output signal of the delay 28 is delivered to a coupler 30. The coupler 30 also receives an input from the coupler 22. Each signal entering the coupler 30 is phased such that the main signal component in each input signal is canceled (or nearly canceled), leaving only an error signal 31 (the distortion created by the main amplifier 20). The coupler 30 outputs the error signal to an error path 32 that includes a gain and phase block 34 and an error or distortion canceling amplifier 36. The error path 32 generates a second error signal 38. The second error signal 38 is a gain and phase adjusted version of the signal 31. At the output, the amplitude of the second signal 38 matches or nearly matches the amplitude of the error component of the signal 21. Also, the second signal 38 is 180xc2x0 out-of-phase with the en-or component of the signal 21.
The signal 21 is output by the coupler 22 to an error delay 40. The error delay 40 provides a time delay approximately equal to the time it takes for the error signal 31 to propagate through the error path 32. The delayed signal from the error delay 40 and the error signal 38 are input to a coupler 42. The two signals are combined in the coupler 42 and, due to the phase and gain adjustments made to each, the error signal 38 substantially cancels the error component of the signal 21, creating an output signal 44 with only the main component of the original input signal 12.
While FFAs provide improved amplification over simple amplifiers, FFAs are still relatively inefficient. Loses in the final coupler (e.g., coupler 42) of an FFA waste a large amount of the error signal. In addition, the distortion amplifier in an FFA requires a relatively high power output capability to prevent the error amplifier from creating a significant level of independent distortion. Accordingly, there is a need for improved methods and amplifiers for amplifying communications signals.
The present invention provides a highly efficient, linear amplifier for communications and other applications in the form of a balanced error correction (xe2x80x9cBECxe2x80x9d) amplifier. (Sometimes xe2x80x9cerror correctionxe2x80x9d and xe2x80x9cerror cancellationxe2x80x9d will be used interchangeably herein.) The BEC amplifier includes a main amplifier operable to receive a main signal and to generate an amplified signal having a main component and an error component. The BEC amplifier also includes a second amplifier coupled in a feed-forward arrangement to the main amplifier and operable to receive an input signal and to generate an output signal having a main component and an error component. A balancing network is coupled to the main amplifier and to the second amplifier. The balancing network isolates a sample of the error component of the output signal of the main amplifier, inverts the sample, and combines the sample with the input signal to the error correction amplifier. A summing point combines the output signal from the main amplifier and the output signal of the error correction amplifier such that the error components of the two output signals substantially cancel one another and the main components of the output signals are added to one another.
The invention also provides a method of providing a first signal path with a gain and a phase, providing a second signal path that is 180xc2x0 out-of-phase to the first signal path and that has a gain that is about two times greater than the gain of the first signal path, providing a third signal path with a gain and a phase, and providing a fourth signal path that has a gain and phase that is substantially the same as the gain and phase of the third path. The method may also include providing a fifth signal path having a gain and a phase, and providing a sixth signal path that is 180xc2x0 out-of-phase to the fifth signal path and that has a gain of about 1.5 times the gain of the fifth signal path.
Because the amplifier and method produce an output signal having a high power level and little or no noise component, spectral regrowth, which is typically analyzed based on the power level of a signal within a desired bandwidth as compared to the power level of signals outside the desired bandwidth, is reduced.
As is apparent from the above, it is an advantage of the present invention to provide an efficient amplifier with improved spectral regrowth. Other features and advantages of the present invention will become apparent by consideration of the detailed description and accompanying drawings.